Abrasive-containing capsular coating composition for pressure-sensitive record paper

ABSTRACT

INERT NON-ABSORPTIVE, RIGID MICROSCOPIC PARTICLES ARE SCATTERED AMONG MICROSCOPIC PRESSURE-RUPTURABLE OIL-CONTAINING CAPSULES IN A COATING ON PRESSURE-SENSITIVE RECORD SHEET MATERIAL.

Patented Mar. 3 9, 119W.

3,573,085 ABRASlVE-CONTAINING CAPSULAR COATING COMPOSITION FORPRESSURE-SENSITIVE RECORD PAPER Glen A. Hemstock, Princeton, N.J.,assignor to Eugelhartl Minerals & Chemicals Corporation, Woodbridge, NJ.No Drawing. Filed July 3, 1968, Ser. No. 742,181 Int. Cl. B41m 5/22 US.Cl. 11736.2 8 Claims ABSTRACT OF THE DISCLOSURE Inert, non-absorptive,rigid microscopic particles are scattered among microscopicpressure-rupturable oil-containing capsules in a coating onpressure-sensitive record sheet material.

BACKGROUND OF THE INVENTION A type of widcly-used pressure-sensitiverecording paper utilizes the reaction between normally colorlesscolorreactant dye material such as a mixture of crystal violet lactoneand benzoyl leuco methylene blue and sensitive pigment material such asattapulgite clay to form the colored mark. An oily solution of the dyematerial is encapsulated in rupturable microscopic capsules usually madeof hydrophilic colloid such as gelatin. The colored mark is formed whenthe capsules are ruptured by printing pressure and the contents of thecapsules come into contact with the reactive pigment particles. Inpresent commercial adaptations of this printing system, one side of thesheet is coated with a profusion of the microscopic capsules and theback of the sheet is coated with attapulgite clay, a colloidal claymaterial. The sheets are stacked so that the capsular coating is inface-to-face relation with the sensitive clay coating on another sheetwhich receives the printed image when the capsules are ruptured.

It has also been proposed (US. 2,730,457 to Barrett K. Green et al.) toprovide the capsular layer with an overlay coating or an underlaycoating of particles of a color-reactant solid such as attapulgite clay.This results in self-contained, pigment-coated record sheet material.

Irrespective of the arrangement that is used, it is apparent that inorder to obtain a clear printed image, the capsules must rupture uponapplication of the printing pressure and the oily content of theruptured capsules must come into adsorptive contact with the sensitivepigment. Failure of the capsules to rupture is especially likely tooccur with self-contained pressure sensitive record material in whichthe capsules are proximate the base sheet and the sensitive pigment iscoated over the capsular coating. With such an arrangement, a printedimage of uniform intensity may not be obtained unless a relatively heavycoating of capsules is employed. Since the microscopic capsules arecostly in comparison to the other constituents of the record material,it is highly desirable to minimize the coat weight of the capsules.

THE INVENTION Stated briefly, in accordance with the present invention,discrete, microscopically-dimensioned, rigid particles of a poorlyoil-receptive material are scatered among individual oil-containingmicroscopic capsules of gelled hydrophilic-colloid material contained asa coating on pressure-sensitive record material. The rigid particles areinert to the oil contained within the capsules and they function torupture the walls of the capsules when the coated paper containing thecapsules is subjected to printing pressure. A coating of sensitivepigment may be provided as an overlay coating (so that pigment isproximate the sheet) or the sensitive pigment may be provided on aseparate sheet. It is also within the scope of the invention toincorporate the sensitive pigment with the capsules and the rigid nonoil-receptive particles so that the three types of particles aresubstantially uniformly interspersed on the sheet and in contact witheach other.

The invention is of special benefit when employed in so-calledself-contained printing sheet material in which the capsular coating isproximate the base sheet and the sensitive pigment is provided as acoating over the capsular coating. As mentioned, difficulty isespecially likely to be experienced in rupturing the capsules in thisform of pressure-sensitive record material.

PRIOR ART I am aware that it has been suggested in US. 2,730, 457(supra) to incorporate sensitive attapulgite clay into a suspension ofencapsulated dye material and to coat the mixture on paper to provide asingle coating. Attapulgite, however, is a colloidal mineral and when itis interspersed wtih the capsules in the coating the clay particlesmerely serve the usual function of a sensitive pigment, i.e., to receivethe printed image when the capsules are ruptured. The attapulgiteparticles do not promote the rupture of the particles since they are toosmall and are not sufficiently hard or abrasive. The present inventionalso differs in concept from the use, suggested in US. 2,665,453, oflarge glass beads or polymer particles in a rupturable film containingprinting fluid for the purpose of providing a smudge-resistant coating.

DESCRIPTION In carrying out this invention, rigid particles Within therange of l to 44 microns, preferably finer than 10 microns, are used asthe rupture-inducing agent. Optimum size within this range depends,inter alia, on the size of the rupturable capsules. The absence ofsubstantial amounts of submicron size or colloidal particles in therupture-inducing agent is preferable. The desired results are notrealized when the rigid particles are too small to rupture the capsulewalls. On the other hand, when the rigid particles are too large therewill be a large spacing between adjacent capsules. Consequently, theresulting printed mark may lack the desired continuity. Withrepresentative capsules having a diameter of about 3 to 10 microns andwalls about 1 micron thick, the use of particles largely within therange of about 1 to 2 microns is preferred.

Another requirement of the rigid particles is that they are white,off-white or transparent.

The rigid particles must have low oil absorption values in order toassure that when the capsules are ruptured the oily contents will bepreferentially adsorbed by the color-reactant sensitive pigmentparticles. The rigid particles should have oil absorption values(Gardner Coleman) of 40 ml./ grams or below, preferably below 20 ml./100 gram. The Gardner Coleman method for measuring oil absorption (ASTMD2813) uses raw linseed oil. The test method is described by Gardner, H.A., and Sward, G. C., Physical and Chemical Examination of Paints,Varnishes, Lacquers and Colors, 9th ed., Inst. Paint Varnish Research,Washington, DC, 1939, page 209.

Rigid particles of various shapes, such as platelets, spheres oracicular form, may be employed.

Rigid, low density, poorly oil-retentive particles are exemplified byemulsified, high-glass transition tempera ture polymers such as highdensity polyethylene and acrylic plastics. These polymers are producedin suitable micron-size particles by emulsion polymerization technique.Products supplied as BS5 acrylic and Poly M which are emulsionpolymerized acrylics and polyethylene, respectively, are examples ofsuitable polymeric particles.

Other particles which may be employed to promote the rupture of thecapsules include glassy frits.

Minerals, exemplified by silica-containing minerals such as chrysotileasbestos shorts, quartz, and ground feldspar may be employed, as may beminerals such as rutile, anatase, calcined bauxite and alkaline earthcarbonate minerals such as calcite and aragonite. Synthetic inorganicsolids such as low surface area aluminas may be used. Hydrate aluminasof the CO30 series are suitable; these are finely divided, whitecrystals of synthetic alpha alumina trihydrate.

The rigid particles are employed in amount within the range of about to300% based on the dry weight of the capsules. The quantity is preferablysutficient to provide at least one rigid particle between adjacentcapsules when the mixture is coated on the sheet.

In putting the invention into practice, the rigid particles may be addeddry or as an aqueous suspension to dry capsules or to an aqueoussuspension (so-called emulsion) of the capsules. It will be noted thatthe suspension of capsules appears to be a milky fluid, althoughexamination in a microscope reveals the presence of the discrete solidcapsules. The capsules and the rigid particles are thoroughly mixed andthe solids adjusted to a suitable level by addition or removal of water,if necessary. An adhesive suspension, usually a coked starch such ascornstarch, is incorporated with the capsular emulsion to produce thecoating composition. Recommended, by way of example, is the use of acoating composition containing to 30% by weight combined capsules andrigid particles and starch in amount within the range of about 5% to 15of the combined weight of the capsules and rigid particles. Optimumsolids and composition of the coating composition will depend on whetherthe coating is applied by an on-machine process or an off-machineprocess.

In order to derive the benefits of the present invention, it ispreferable, but not necessary, for the rigid particles to be in directcontact with the unruptured capsules with which they are interspersed onthe coated sheet. It is essential, however, that the rigid particles bein sufficiently close proximity to the capsules so that the rigidparticles are in surface-to-surface contact with adjacent capsules afterthe capsules have been deformed by printing pressure. In order to assurethe required proximity of the capsules and rupture-inducing rigidparticles, the coating composition should be applied to the base sheetat an adequate coat weight.

By employing the rigid particles as an additive, the coat weight of thecapsular coating may be reduced from the amounts usually employed sincemore effective use is made of the capsules. Coat weights within therange of 1 to 5 lb./ream (3300 ft?) are suitable.

The base sheet may be conventional chemical wood pulp stock, typicallyabout 15 to 50 lb. basis weight. When making self-contained recordmaterial it is preferable to use a base sheet that has been hornifiedand smoothed by passage through wet-calender rolls provided with waterboxes on the stacks of upper jack rolls, as described in my copendingapplication, Ser. No. 675,793, filed Oct. 17, 1967, now abandoned.

After the capsular coating has been applied and dried, the sheet may beprovided with an overlay coating of sensitive pigment such asattapulgite clay as described in US. 2,730,457, calcined partiallyrehydrated kaolin, as described in my patent, US. 3,226,252, or akaolin-acidic polymeric resin such as a phenylphenol-formaldehyde resin,as described in Netherlands application 6,511,224. It will be noted thatthe aforementioned sensitive pigment materials have higher oilabsorption values than the abrasive inert particles which are containedin the capsular coating layer.

As mentioned above, the present invention is of special benefit whenapplied to the production of pressure-sensitive record material in whichthe capsular coating is proximate the base sheet and the coating ofsensitive pigment is superimposed on the capsular coating. However, itis also within the scope of the invention to apply the coating ofsensitive pigment to sheet material and employ the capsular coatingmixture as an overlay coating. Also, the coating of sensitive pigmentmay be applied to the reverse side of a capsule-coated sheet and theresulting sheet may be used with another sheet coated with sensitivepigment in a manner such that the coating of sensitive pigment is inface-to-face relationship with the capsular coating. As anotheralternative, the sensitive pigment may be incorporated with the rigidparticles and capsules in a single coating composition containingsufficient sensitive pigment to provide a smooth opaque finish to thepaper and to assure continuity of the printed image. When the pig mentis incorporated with the capsules and rigid particles in a singlecoating, a coat weight within the range of 3 to 10 lbs. per ream isrecommended.

EXAMPLE Equal weight proportions of crystal violet lactone (3,3 bis(p-dimethylaminophenyl) o-dimethylamino phthalide) and benzoyl leucomethylene blue are dissolved in trichlorodiphenyl to form a /2 (weight)solution. A sol containing 10% of pigskin gelatin is separatelyprepared. Twenty parts by weight of the oil solution is added to partsby weight of dispersed gelatin with stirring, forming an oil-in-wateremulsion with the sol, as described in Reissue 24,899 to B. K. Green.Coacervation is induced, and the coacervate is gelled and hardened withformaldehyde, as described in said patent. The solids content of theemulsion is then adjusted to 50% by weight.

To 100 parts by weight of the 50% suspension of capsules, 20 parts byweight of a 50% aqueous suspension of emulsion-polymerized, high densitypolyethylene is added. The polyethylene is in the form of smallspherical particles within the range of 1 to 5 microns and having aspecific gravity of 0.95. The suspensions are mixed thoroughly and 40parts by weight of a 10% suspension of cooked cornstarch is added. Thesuspensions are thoroughly mixed and diluted to 20% solids with water.

The coating composition, which resembles a creamy paste, is applied to35 lb. per ream (25" x 40":500 sheets) bleached chemical wood cellulosefiber stock with an air-knife coater. Coat weight is 3 lb. per ream. Thecoating is dried and provided with a 1 lb. per ream overlay coating of amixture of Attasorb RVM and Syloid by the procedure and with theingredients described in my patent, U.S. 3,226,252.

It will be understood that the foregoing example is given forillustrative purposes only and the constituents of the capsule coatedsheet may be varied within the scope of the appended claims.

I claim:

1. Pressure-sensitive record material comprising a sheet of paper onwhich is a coating of (a) discrete, microscopic pressure-rupturablecapsules of oil-impermeable gelled hydrophilic colloid materialcontaining therein an oily solution of dye-forming material, anduniformly interspersed among said capsules and in proximity thereto (b)discrete rigid particles of a solid material having a size within therange of 1 to 44 microns and a Gardner- Coleman oil absorption valuebelow 40 ml./100 grams, said particles (b) being inert towards said oilydye-forming content of said capsules (a) when in contact therewith andfunctioning to rupture the walls of the capsules when the coated papercontaining the capsules is subjected to printing pressure, saidparticles (b) being present in the amount of about 5-300% based on theweight of the capsules, said coating having a weight within the range of1 to 5 lbs/ream.

2. The record material of claim 1 wherein said particles (b) are finerthan 10 microns.

3. The record material of claim 1 wherein said particles (b) compriseglassy frit.

4. The record material of claim 1 wherein said particles (b) comprise asilica-containing mineral.

5. The record material of claim 1 wherein said particles comprise analkaline earth carbonate mineral.

6. The record material of claim 1 wherein said particles are composed ofaluminum oxide.

7. The record of claim 1 wherein said particles (b) are composed of anemulsified high glass transition 10 References Cited UNITED STATESPATENTS MURRAY KATZ, Primary Examiner US. Cl. X.R.

